This proposal examines how the regulation of adult neurogenesis produced by repeated antidepressant drug treatment in mice may contribute to behavioral recovery from stress and depression. Major depressive disorder results from convergent underlying genetic predispositions and exposure to environmental stress. Evidence for increases of neurogenesis in the adult dentate gyrus of the hippocampus is a model for changes in neuroplasticity that emerge with chronic treatment with antidepressant drugs and serve as a model for the production of therapeutic effects. Most studies have measured the effects of antidepressants using rodents under routine housing conditions and have ignored the critical role of genetics and stress in contributing to drug treatment effects. We have identified a mouse strain (MRL/MpJ) that shows larger increases in neurogenesis following chronic fluoxetine. Also, the addition of stress hormones augments the effects of fluoxetine on neurogenesis in a non-responder mouse strain (C57BL/6). The central hypothesis of this grant proposal is that genetic predispositions and stress produce a critical role for revealing the effects of chronic antidepressant treatments on hippocampal neurogenesis. The use of flow cytometry as a technique to measure neurogenesis rapidly developed by this laboratory will enable completion of the large number of studies required for investigation of the pharmacology of neurogenesis. The primary goals of this proposal are: 1) demonstrate the critical role of exposure to corticosterone (CORT) in augmenting responses to chronic treatment with antidepressant drugs on neurogenesis and behavior in a responder and non-responder mouse strain; 2) show whether environmental stress plays a similar role in causing antidepressant drugs to alter neurogenesis and behavior; and 3) determine whether hippocampal BDNF serves as a mechanism underlying the production these effects by chronic antidepressant drug treatments.